In certain processes for the formation of the conductive gates used to control the N channel and P channel transistors which make up CMOS devices, a polycrystalline silicon material is deposited on a gate oxide to form a portion of each transistor's gate. Following the formation of the polycrystalline silicon, a layer of tungsten silicide (WSi.sub.x) is deposited on the polycrystalline silicon to form a metal strapping to reduce the overall impedance of the gate structure (and thus increase the speed of the device). Typically, the tungsten silicide is formed by chemical vapor deposition using tungsten hexafluoride (WF.sub.6) and silane (SiH.sub.4) . The tungsten silicide that deposits on the polycrystalline silicon contains fluorine atoms that upon annealing migrate into the polycrystalline silicon and the underlying gate oxide.
Unfortunately, some of the fluorine atoms also displace oxygen atoms in the underlying gate oxide (SiO.sub.2) and destroy the gate-oxide bonding structure. First, some of these displaced oxygen atoms may enter into the silicon underlying the gate oxide. These oxygen atoms then combine with the silicon underlying the gate oxide to cause the gate oxide to become thicker. As a result, the process of forming the tungsten silicide metal strapping on the gate electrode destroys the integrity of the original gate oxide and thus changes the performance characteristics of the resulting device. For example, the thickening of the gate oxide may uncontrollably, and thus undesirably, change (usually increase but sometimes decrease) the threshold voltage of the resulting device. A change in the threshold voltage of the resulting device is undesirable because it increases yield loss and makes it difficult to predict the performance of the resulting circuits formed using the resulting device. In general it is undesirable to have a threshold voltage for an MOS device which is other than predicted from design considerations. Second, the altered gate-oxide structure will have much degraded charge-to-breakdown and breakdown voltage characteristics.
The prior art did not address these problems. The prior art was directed solely to depositing a WSi.sub.x metal strapping onto the polycrystalline silicon layer and made no attempt to correct the device degradation problems. Although WSi.sub.x physical vapor deposition (such as sputtering deposition) can have no fluorine problem, because of its poor step-coverage this WSi.sub.x -method is less preferred than the chemical vapor deposition method.
The prior art has several disadvantages. For example, (1) the prior art does not maintain the gate-oxide integrity, i.e., it has a lower charge-to-breakdown and breakdown voltage. (2) The final gate oxide is thicker than desired; thus the threshold voltage is uncontrollable and unpredictable across process runs. And (3) when the process is performed ex-situ (breaking the vacuum of the semiconductor fabrication chamber after forming the gate-oxide and polycrystalline silicon regions), an extra step is required to clean the native oxide that forms on the semiconductor device before forming the strapping on the polycrystalline gate.